Aneurysms are abnormal dilatations of blood vessels. A critical type of aneurysm is a brain aneurysm which is an abnormal dilatation of one of the blood vessels supplying blood flow to the brain itself. Aneurysms are felt to start as an area of potential weakness in the wall of the artery and typically occur at an arterial branch point. Over time, the natural pulsation of blood against the area of weakness can gradually cause the weakened area to dilate or enlarge. This process may be analogized to inflating a balloon. As the balloon stretches, the wall of the balloon typically becomes thinner. Eventually, if the wall is stretched too thin, the balloon bursts.
Similarly, if an aneurysm becomes too thin, the wall bursts allowing blood to escape from the blood vessels, in particular, the arteries in which the blood is supposed to be contained. The blood spills into the subarachnoid space, a potential space surrounding the brain that is normally filled with cerebrospinal fluid. This aneurysm bleeding or rupture results in what is known as a subarachnoid hemorrhage (SAH). Unfortunately, despite best medical efforts, SAH will be fatal in roughly 50% of the cases, with a significant percentage of patients dying before they even reach the hospital. Of survivors, approximately one half are left with permanent disability, for example, a stroke, which permanently compromises their independence and quality of life. Once an aneurysm ruptures, if it does not stop bleeding on its own within seconds, the patient generally dies. After an aneurysm has bled once, the risk of rebleeding is very high, and each hemorrhage carries at least a 50% mortality rate so urgent treatment is generally performed to prevent rebleeding. In addition, aneurysms are now being found prior to rupture as sophisticated imaging techniques such as magnetic resonance imaging, magnetic resonance angiography, and computerized tomographic angiography are gaining widespread use. These non-invasive techniques allow for the demonstration of an aneurysm before it bleeds, and their increasing use has led to the discovery of an unexpectedly significant number of patients with unruptured, asymptomatic lesions.
In general, there are two forms of treatment for a brain aneurysm, whether it has bled or not. Open microsurgery, a so-called craniotomy, with aneurysm clipping entails opening the skull and placing a metal clip, usually made from titanium, across the neck of the aneurysm to reconstruct the normal contour of the parent artery which harbors the aneurysm. Open clipping procedures have been performed for almost a century. This is the traditional, so-called “gold standard” method for treating an aneurysm of the brain. Once properly clipped, aneurysms rarely recur, and the patient is generally cured. If the aneurysm has already bled, there may be consequences of the initial hemorrhage, but the aneurysm cannot bleed again, and the patient is thus protected from the insult of a repeat hemorrhage. Over the past 50 years, repeatedly refined designs of increasingly pure titanium clips have been manufactured providing the neurovascular surgeon with numerous clip shape and size choices that can either singly or in combination address most, but not all, aneurysms.
The major alternative form of treatment for brain aneurysms is endovascular coiling. Coiling is a much newer technique that was introduced in the 1990's. A microcatheter is fed from a puncture site in the femoral or groin artery up into the carotid or vertebral arteries, and all the way into the aneurysm itself at the base of the brain. This is all done while watching the progress of the microcatheter on a fluoroscope using biplane digital subtraction angiography. Once the catheter tip is positioned inside the aneurysm, a tiny platinum wire is fed through the catheter into the aneurysm. The wire coils up on itself and fills the aneurysm so that when completed, the coiled aneurysm is essentially full of wire and no blood can enter the aneurysm from the main parent artery. This coil mass essentially acts as a physical barrier preventing blood from entering the aneurysm and reaching the wall of the aneurysm. Thereby, the aneurysm is prevented from rupturing.
Coiling of aneurysms has rapidly gained acceptance as an important technique in aneurysm treatment. It is minimally invasive, avoiding the need for an open brain operation, i.e. it avoids a craniotomy. The patients tend to recover more quickly and have less down time after the procedure compared with open surgery. Unfortunately, coiling works best for aneurysms with a narrow neck that will help prevent the coil mass from herniating back into the main artery, i.e. the parent artery. In wider necked aneurysms, such coil protruding into the main artery could result in blockage of blood supply through the main artery and subsequent stroke. In addition, many aneurysms will recur or regrow after coiling. As the blood pumps against the coil mass sitting in the aneurysm, it tends to “compact” the coils themselves out towards the dome of the aneurysm, and then blood flow can once again enter the aneurysm, and the patient is again at risk for bleeding from the thin wall of the aneurysm. Nevertheless, the great appeal and relative ease of coiling as opposed to open surgery has resulted in a substantial percentage of aneurysms being treated with endovascular coiling.
In general, aneurysm clips are comprised of two blades or arms that are parallel and opposed to each other in the resting position. A clip applying device which opens the clip blades is used to allow the clip, which can be opened and closed like a clothespin, to be applied to the aneurysm. As the clip is allowed to close, the blades return to the parallel, perfectly opposed position, collapsing the soft aneurysm between the blades. When applied properly down at the base of the aneurysm neck, the blades completely stop any blood flow into the now “empty sac” of the aneurysm which has been cured. When clipping an aneurysm, the surgeon relies on the softness of the wall of the aneurysm which allows the clip blades to close in opposition to one another. Anything that prevents the clip itself from closing properly may prevent proper clipping of an aneurysm. Because the blades of the clip have a strong so-called “closing strength” a force will be exerted against any such an obstructing item as the blades attempt to resume their resting, closed position. Those aneurysms that are not soft-walled represent the crux of the problem addressed by this development.
An aneurysm may not allow a clip to close properly because there is atheroma or calcium, which forms a hardening of the artery within the wall of the aneurysm itself, because there is organized hematoma or thrombus within the aneurysm itself, or because the aneurysm is filled with coils from a previous endovascular treatment. When traditional clips fail because of non-compliance of the aneurysm, a dangerous situation is created. The clip can slip back off the aneurysm, it can be forced down onto the neck of the aneurysm blocking flow through the main artery or its branches, or it can tear through the wall of the aneurysm with disastrous consequences. As coiling has become more common, and as coiling has been used increasingly for small, simple aneurysms, those aneurysms that are referred for open surgery are increasingly large and giant aneurysms with atheromatous, calcified walls, intraluminal thrombus, and previously coiled aneurysms in which the coils have failed. These are precisely the subgroups of aneurysms that are hardest to treat using conventional clip technology, for the reasons stated above. A better option for surgical aneurysm obliteration is clearly needed for these most difficult and challenging lesions.
As described above, aneurysms are generally treated with either clipping or coiling. The large and giant aneurysms are the ones most likely to have thick non-compliant walls, and these aneurysms represent a serious management challenge. Also, the growing number of previously coiled aneurysms that have failed and recurred is increasing exponentially. There are no optimal treatments for these lesions today. Some of these aneurysms can be clipped using very large, long clips that have a very high closing pressure. The aneurysms can be “crushed” with a forceps or clamp first to allow the clips to close, but this is a dangerous maneuver that can rupture the aneurysm or shower clot out of the aneurysm into the blood supply causing a stroke. Some aneurysms can be treated by temporarily stopping blood flow to the aneurysm, cutting the aneurysm open, and removing the thicker portion of the wall with any associated thrombus. The aneurysm can then be closed by over-sewing the opening, so-called aneurysmorrhaphy, or with very large clips. Unfortunately, stopping the blood flow to the aneurysm, even temporarily, may mean stopping the blood flow to the normal brain which can cause a stroke. Even if no stroke occurs from the temporary arterial occlusion, the wall of the aneurysm may be brittle or friable and may not be repairable once opened resulting in disaster.
Still other aneurysms require permanent occlusion of the entire involved artery, relying on collateral blood supply to prevent a stroke or performing a delicate brain bypass surgery to bring new blood supply to the part of the brain that was being fed by the artery that must be sacrificed. If the bypass is unsuccessful, the result is usually a severe stroke. Some aneurysms can be re-filled with coils, but many will only regrow yet again over time, at which point the problem may be even worse. Finally, some aneurysms can be wrapped with gauze to toughen the wall and decrease future risk of bleeding. This is the least reliable way to treat an aneurysm, and there is little data on long-term follow-up after wrapping. The aneurysm is essentially left unsecured with the potential for future bleeding.
In short, there are no good treatment methods currently available for these vexing lesions.
As described in detail above, all current treatment options are a high risk and dangerous. The complication rates including stroke and death rates in these patients are many times higher than in the patients with simple smaller aneurysms. This is simply a reflection of the limited available technology to treat these lesions. Because there is nothing better available, surgeons are forced to apply a technology that is not designed to treat properly these lesions in particular. Clips which depend on a soft, compliant aneurysm wall to close are generally not intended for and are no match for a giant, atheromatous aneurysm or an aneurysm full of coils.
Therefore, any new method designed to treat this subgroup of aneurysms will have to address the wall non-compliance that prevents traditional clips from closing properly. It should be able to address the problems of a thick wall as well as a mass of coils within the aneurysm, both of which will work against the closure of all standard aneurysm clips.